Muscarinic cholinergic receptors mediate the actions of the neurotransmitter acetylcholine in the central and peripheral nervous systems. Muscarinic receptors play a critical role in the central nervous system mediating higher cognitive functions, as well as in the peripheral parasympathetic nervous system where they mediate cardiac, respiratory, digestive, and endocrine and exocrine responses. Five distinct muscarinic receptor subtypes have been identified, M1-M5. The muscarinic M1 receptor subtype is predominantly expressed in the cerebral cortex and is believed to be involved in the control of higher cognitive functions; the M2 receptor is the predominant subtype found in heart and is involved in the control of heart rate; the M3 receptor is widely expressed in many peripheral tissues and is believed to be involved in gastrointestinal and urinary tract stimulation as well as sweating and salivation; the M4 receptor is present in brain and may be involved in locomotion; the M5, receptor is present in the brain where its role is at present poorly defined. M1 and M4 have been particularly associated with the dopaminergic system.
Conditions associated with cognitive impairment, such as Alzheimer's disease, are accompanied by a reduction of acetylcholine content in the brain. This is believed to be the result of degeneration of cholinergic neurons of the basal forebrain, which widely innervate multiple areas of the brain, including the association cortices and hippocampus, that are critically involved in higher processes.
Efforts to increase acetylcholine levels have focused on increasing levels of choline, the precursor for acetylcholine synthesis, and on blocking acetylcholineesterase (AChE), the enzyme that metabolizes acetylcholine. Attempts to augment central cholinergic function through the administration of choline or phosphatidylcholine have not been successful. AChE inhibitors have shown therapeutic efficacy, but have been found to have frequent cholinergic side effects due to peripheral acetylcholine stimulation, including abdominal cramps, nausea, vomiting, and diarrhoea. These gastrointestinal side effects have been observed in about a third of the patients treated. In addition, some AChE inhibitors, such as tacrine, have also been found to cause significant hepatotoxicity with elevated liver transaminases observed in about 30% of patients. The adverse effects of AChE inhibitors have severely limited their clinical utility.
The dopamine hypothesis of schizophrenia suggests that increased dopamine neurotransmission underlies the positive symptoms of the disease and is supported by the evidence that dopamine receptor blockade is effective in ameliorating such psychotic symptoms. Further, drugs that enhance dopamine neurotransmission in the brain cause psychotic-like episodes in man and exacerbate psychotic symptoms in schizophrenic patients. In animal studies, drugs that increase dopamine neurotransmission cause behavioural effects such as increased locomotion, climbing and deficits in prepulse inhibition. Known antipsychotics and dopamine receptor antagonists can block these behavioural effects. Unfortunately, dopamine receptor antagonists also cause severe extrapyramidal side effects in patients as predicted by induction of catalepsy in animal models. These extrapyramidal side effects include tremor, bradykinesia, akithesias, and tardive dyskinesias.
Due in part to these observations, the discovery of agents with M1 receptor agonist activity has been sought after for the treatment of dementia. However, existing agents lack specificity in their actions at the various muscarinic receptor subtypes. Known M1 muscarinic agonists such as arecoline have also been found to be weak agonists of M2 as well as M3 receptor subtypes and are ineffective in the treatment of cognitive impairment, due in large part to their dose-limiting M2 and M3 receptor mediated side effects.
Xanomeline (Shannon et al., J. Pharmacol. Exp. Ther. 1994, 269, 271; Shannon et al., Schizophrenia Res. 2000, 42, 249) is an M1/M4 preferring muscarinic receptor agonist with little or no affinity for dopamine receptors despite inhibiting A10 but not A9 dopamine cells. The thiadiazole derivative PTAC has been reported (Shannon et al., European Journal of Pharmacology, 1998, 356, 109) to have partial agonist effect at muscarinic M2 and M4 receptors and antagonist effect at muscarinic M1, M3, and M5 receptors as well as exhibiting functional dopamine antagonism.
Recently, muscarinic agonists including xanomeline have been shown to be active in animal models with similar profiles to known antipsychotic drugs, but without causing catalepsy (Bymaster et al., Eur. J. Pharmacol. 1998, 356, 109, Bymaster et al., Life Sci. 1999, 64, 527, Shannon et al., J. Pharmacol. Exp. Ther. 1999, 290, 901, Shannon et al., Schizophrenia Res. 2000, 42, 249). Further, xanomeline was shown to reduce psychotic behavioural symptoms such as delusions, suspiciousness, vocal outbursts, and hallucinations in Alzheimer's disease patients (Bodick et al., Arch. Neurol. 1997, 54, 465), however treatment induced side effects that severely limit the clinical utility of this compound.
Analogues of 1,2,5-thiadiazole have been reported (Sauerberg et al., J. Med Chem. 1998, 41, 4378) to have high affinity and selectivity for central muscarinic receptors as well as exhibiting functional dopamine antagonism despite lack of affinity for dopamine receptors.
The present investigators have, in part, focussed their efforts on the development of molecules that simultaneously reduced the positive symptoms and improved the negative symptoms and the cognitive impairments associated with schizophrenia as a novel treatment of mental disorders. It is the intent of the present investigators to demonstrate that muscarinic M1 and/or M4 agonists with combined D2 antagonist activity may possess superior antipsychotic efficacy without the side effects associated with high dose D2 antagonism alone. The D2 antagonist properties of some of the compounds of the present invention may contribute to a reduction in the positive symptoms of this disease.
Based on distribution of M1 and M4 receptors in the cerebral cortex and hippocampus (the areas involved in higher order cognitive functions), the M1 and/or M4 agonist properties of these compounds may reduce the cognitive dulling and perhaps ameliorate other negative symptoms associated with schizophrenia. (Friedman, Biol. Psychiatry, 1999, 45, 1; Rowley, J. Med. Chem. 2001, 44, 477; Felder, J. Med. Chem. 2000, 43, 4333). This unique combination of central nervous system activities in one molecule is unprecedented and may lead to the development of an entirely new class of antipsychotic drugs, ones with the superior clinical properties without the limiting side-effect profile.
U.S. Pat. No. 3,324,137 and U.S. Pat. No. 3,365,457 describe N-[indolyl-lower-alkanoyl]-1,5-iminocycloalkanes and iminocycloalkanes not encompassed by the invention.
EP 0 584 487 describes 4,5-dihydo-4-oxo-pyrroles with linked to piperazine rings not encompassed by the invention.
Mokrosz et al (Pharmazie, 52, 1997, 6, p 423) describes N[3-(4-aryl)-1-piperazinyl)propyl] derivatives of indolin-2(1H)-one, quinolin-2-(1H)-one and isoquinolin-1-(2H)-one, which are not encompassed by the invention.